The present invention relates to a memory control device incorporated in a reproducing apparatus adapted, for example, for playing a disk recording medium and for controling a data writing operation of storing data that have been read from the disk recording medium in a memory where the data are to be stored temporarily.
CD players are widely diffused in use as apparatus for reproducing audio data from a CD (Compact Disk) and outputting the reproduced data.
In a CD player, if some external disturbance such as a shock or a vibration is given thereto, it causes disorder of tracking and focus servo control, for example, and consequently generates error in reading the data. Practically, such error induces interruption of the reproduced audio signal.
In view of this problem, there are now widely available some improved CD players furnished with a shock-proof function which exhibits resistance against the external disturbance mentioned.
As is well known, in the structure of such a CD player equipped with a shock-proof function, a buffer memory is provided for temporarily storing the reproduced data obtained from a CD. The data are read out from a CD at a two-fold or higher reproduction speed, for example, and the data are written in the buffer memory at a data rate corresponding to the reproduction speed. After storage of the data by more than a predetermined quantity in the buffer memory, the data are read out from the buffer memory at a one-fold reproduction speed.
Since the data write speed to the buffer memory is higher than the read speed as described, it is customary that the data reproduction from the CD and the data writing into the buffer memory are paused when more than a predetermined quantity of the data have been written, so as not to cause overflow of the data in the buffer memory. Meanwhile, the data are read out continuously from the buffer memory. When the data stored in the buffer memory have been reduced below the predetermined quantity, the data reproduction from the CD and the data writing into the buffer memory are resumed. That is, in the reproducing apparatus equipped with a shock-proof function, reproduction of the data from the CD and writing of the reproduced data into the buffer memory are performed intermittently.
As long as the data thus reproduced intermittently from the CD are written intermittently into the buffer memory as described, it is necessary to maintain continuity of the time base information between the data written finally into the buffer memory and the data to be written thereafter into the buffer memory. If this continuity fails to be maintained, it is impossible to obtain a requisite link of the reproduced audio outputs. A control action executed for this purpose is termed sound link control or data link control, for example.
For such sound link control, channel-Q subcode data is used as a subcode inserted in audio data recorded on a CD.
As is known, CD-format audio data are composed of EFM frames, each serving as a minimum component unit, and 98 EFM frames are grouped to form one subcoding frame. Each subcoding frame is updated at an interval of {fraction (1/75)} second.
A subcoding frame has subcodes of eight channels P, Q, R, S, T, U, V and W. Time base information of the audio data recorded on the CD is recorded in the channel-Q subcode data (hereinafter referred to as subQ data). The sound link control is executed by using the time base information represented by the subQ data.
FIGS. 12A to 12E conceptually show how writing into a buffer memory is controlled on the basis of such subQ data.
First, FIG. 12A shows a signal SCOR generated in a reproducing apparatus. Synchronizing signals S0 and S1 called subcode sync are inserted in a subcoding frame. This SCOR is a signal generated at the timing of detection of at least either the subcode sync S0 or S1. In the reproducing apparatus, update of the subQ data is recognized in response to the signal SCOR. Although not shown here, error detection data (CRC) relative to the subcode data is detected in the reproducing apparatus, thereby generating a signal CRCF which signifies whether or not the subQ data in the relevant subcoding frame is read properly.
FIG. 12B shows a signal GRSCOR. This signal GRSCOR is generated only in a steady generation state where a signal SCOR can be obtained continuously in synchronism with the timing of 98 EFM frames, e.g., at the timing delayed from SCOR by 92 EFM frames.
In the reproducing apparatus, the subQ data is read out in synchronism with generation of the signal SCOR, and a decision as to whether or not to execute writing into the memory in accordance with the content of the read out subQ data is made prior to the timing of the first signal GRSCOR generated after reading the subQ data. A control command is issued in accordance with the result of such a decision.
Supposing now that the data to be written into the buffer memory are such as those shown in FIG. 12C, if signals SCOR and GRSCOR are generated steadily at proper timings as shown in FIGS. 12A and 12B, then the data are written into the buffer memory sequentially as data D1xe2x86x92D2xe2x86x92D3 . . . in synchronism with the signal GRSCOR, as shown in FIGS. 12C and 12D. In this case, writing is performed at a predetermined data rate higher than a one-fold speed for example.
When the data thus written are read out, the data are reproduced with the time-series continuity maintained in the order of data D1xe2x86x92D2xe2x86x92D3 . . . as shown in FIG. 12D. In this case, the data are read at a rate corresponding to a one-fold speed.
It is generally known that, if any undesired condition with unstable servo control or the like is induced by some external disturbance given to the reproducing apparatus or due to some flaw or dust on the disk, the following abnormal states occur with regard to the signal SCOR which is a reference timing signal for detection of the subQ data.
One state relates to xe2x80x9cSCOR dropoutxe2x80x9d which signifies that a signal SCOR fails to be generated at a proper timing. In this state, a signal SCOR drops out at a certain timing as shown in FIG. 13A for example. In this case, a signal GRSCOR shown in FIG. 13B is generated stably, and a signal CRCF shown in FIG. 13C also holds its high (H) level, so that the subQ data are read out properly.
To the contrary, there may occur a state of xe2x80x9cfalse SCORxe2x80x9d where a signal SCOR is generated at an improper timing, as shown in FIG. 14A for example. Here, a signal GRSCOR is outputted steadily as shown in FIG. 14B, but a signal CRCF of FIG. 14C is turned to its low (L) level at the timing of generation of a false SCOR, so that the subQ data corresponding to the false SCOR is not read out correctly.
Although unshown, there may further occur another state of xe2x80x9csubQ data dropoutxe2x80x9d where the subQ data fails to be read out even with generation of the signal SCOR.
In case the three states mentioned above have occurred, if such states are merely transient as shown in FIGS. 13 and 14, data writing into the buffer memory and sound link control are executed substantially correctly in most cases. However, if at least one of these three states is continuous to a certain extent, then it becomes impossible in the reproducing apparatus to make a proper decision as to whether xe2x80x9cSCOR dropoutxe2x80x9d or xe2x80x9cfalse SCORxe2x80x9d has really occurred or not. As a result, it induces malfunction such that the data to be written essentially into the buffer memory are not written, or the data not to be written therein are written erroneously.
Assume here that, in a normal state, data D1 to D7 are written successively into the buffer memory as shown in FIG. 15A. If the aforementioned malfunction is caused, data writing into the buffer memory may be performed in such a manner that, for example, the data D3 to be written essentially drops out as shown in FIG. 15B. Another example is such as shown in FIG. 15C where the data D3 is written in duplicate. More specifically, the data D4 needs to be written next to the preceding data D3 in a normal state, but actually the data D3, which is not to be written at this time point, has already been written therein.
If such erroneous data writing link is executed, the reproduced data output comes to have an incorrect waveform shown in FIG. 15B or 15C instead of a correct waveform shown in FIG. 15A, hence impairing the data reproduction fidelity.
In view of the problems described above, it is an object of the present invention to provide an improved memory control device which enables proper writing of reproduced data from a recording medium into a buffer memory despite some instability of the reproduction state, thereby ensuring satisfactory fidelity of the reproduced data.
According to one aspect of the present invention, there is provided a memory control device capable of controlling data writing into a memory means where the data reproduced from a disk recording medium are stored temporarily.
The memory control device comprises a signal generation means for generating a stable sync detection signal which is obtained at a normal detection timing of a synchronizing signal in response to a sync detection signal generated by detection of the synchronizing signal inserted in the reproduced data per predetermined data unit. The memory control device further comprises a data writing control means capable of reading out time base information inserted per predetermined data unit in synchronism with the timing of generation of the stable sync detection signal, and also capable of controlling the data writing into the memory means on the basis of the time base information thus read.
In this structure, a stable sync detection signal which is generated substantially at the same timing as that of a normal sync detection signal and is maintained in a better state of generation steadier than that of the sync detection signal can be generated. Due to execution of reading the time base information on the basis of the generation timing of such stable sync detection signal, it becomes possible to read out the time base information always at the proper timing regardless of some unsteady state of the actual sync detection signal.